In recent years, biodegradable resins typified by polylactic acid have come into focus from the viewpoint of environmental preservation. The polylactic acid is one of the most heat-resistant and highly transparent biodegradable resins, and is less expensive because of its mass producibility. Therefore, the polylactic acid is very useful. However, the polylactic acid has an insufficient gas barrier property for use as a material for fluid storage containers such as cosmetic containers and food storage containers, and is liable to be deformed during use at higher temperatures. Therefore, the polylactic acid has limitations in its applications.
One exemplary method for improving the gas barrier property of the biodegradable resin is to use a phyllosilicate in combination with the biodegradable resin. In Japanese Journal of Polymer Science and Technology Vol. 59, No. 12, 2002, pp. 760-766, it is reported that the oxygen permeability coefficient of the resin is reduced to 54 ml·mm/m2·day·MPa by using 7.5 wt % of a swellable phyllosilicate organically treated with a specific ammonium ion based on 100 wt % of the resin. However, even if the phyllosilicate is added in an increased amount, the improvement in the gas barrier property is peaked out. No consideration is hitherto given to a method for further improvement of the gas barrier property. Therefore, it is impossible to impart the biodegradable resin with a gas barrier property sufficient for use as a material for the containers.
On the other hand, a crystalline polymer can be imparted with a gas barrier property by increasing the crystallinity thereof. However, the polylactic acid has a lower crystallization speed. Even if the temperature of a mold is set at a temperature of 90 to 120° C. which is optimum for crystallization, the polylactic acid is in a semi-molten state. When the mold temperature is set at around a room temperature, the polylactic acid is cooled to be solidified, but its crystallinity is extremely low (e.g., Plastics Vol. 53, No. 10, 2002, pp. 37-39). To cope with this, JP-A-9-25345 discloses a molded product of a polylactic acid resin having a crystallinity increased by drawing. However, the polylactic acid resin crystallized by the drawing has insufficient heat resistance.
JP-A-2003-253009 discloses a method for increasing the crystallinity by heat-treating a product molded from a polylactic acid resin containing talc before or during the molding. However, the heat resistant temperature (DTUL) of the crystallized resin containing talc is about 100° C., so that the resin cannot withstand the heat treatment for a long period. To impart the resin with the same level of heat resistance, talc should be added in an amount of not less than 20 wt %. This reduces the moldability, and increases the specific gravity. In JP-A-2003-128901, the crystallization speed is increased by cross-linking polylactic acid with a (meth)acrylate compound. However, the physical properties and appearance of the resulting product are not satisfactory.
JP-A-2001-164097 discloses a hollow container molded from a composition containing an aliphatic polyester and an organic clay, but no consideration is given to improvement of the gas barrier property by thermal crystallization.
WO03/008178A1 discloses a product produced by molding, drawing and thermally fixing a hydroxyalkanoate resin consisting essentially of polylactic acid. However, an ordinary polylactic acid is used, so that the crystallization speed is lower. Therefore, it is difficult to cause the crystallization to proceed in a high temperature mold.